Firefox is one of the best and most modern browsers out there today. It has a great plug-in / extension interface and is very popular for its extensions.
I'm not going to talk about the advantages or which browser is better right now. This is a decision everyone has to make for themself.

I don't claim this list to be complete or right or anything. A better name would probably be "my favourite Firefox extensions" but that just doesn't sound as much as a superlative.

The best Firefox extensions:

• Web developer toolbar The must-have for everyone doing simple to complex web sites.
• CSSViewer New and hot: shows CSS properties live on the page while you hover an element
• Google pagerank status - No matter if it's even right, you want to know: Google Pagerank also for Mozilla and Firefox.
• Bookmarks synchronizer - Synchronizing bookmarks between multiple user accounts, Firefox installs, or computers.
• Mouse gestures - Idea stolen from Opera. You will miss it once you use a browser without gesture recognition. You will move your mouse up, and up, and up. But no new tab/window will open.
• SessionSaver - Keeps record of your open windows and tabs once you close/crash Firefox
• Nightly tester tools - Firefox devs like to change the API and make extensions become incompatible. Use this until the extensions have been made compatible.
• Fasterfox - Speed up your connection and display page load time. Good to know if a page just "feels" slow/fast or really is.
• Auto copy - Copy stuff to the clipboard by simply highlighting, no more ctrl+c required. Just like Trillian or others.

You know any other useful extensions? Let me know.